Continuous liquid flow system

ABSTRACT

The disclosure relates to a system for continuous delivery of a measured flow of liquid to a processing apparatus comprising a reservoir ( 10 ) for holding a bulk supply of the liquid and an intermediate vessel ( 12 ). A first pump delivers liquid form the intermediate vessel to processing equipment at one rate. A second pump ( 13 ) delivers liquid from the reservoir to the intermediate vessel of a second much faster rate. A monitor device ( 14 ) continuously monitors the weight of the vessel and controls the first pump accordingly to deliver the required weight of liquid at the required rate to the processing equipment. The monitoring device determines when the weight of liquid in the intermediate vessel has reached a minimum position to initiate operation of the second pump ( 13 ) to refill the vessel and a further device terminates operation of the second means when the liquid level in the vessel reaches a maximum position.

[0001] The present invention relates to a method of continuouslymeasuring the flow of a liquid. Pumping liquids often results in thegeneration of bubbles which make it difficult to measure and controlwith conventional flow measuring devices.

[0002] In the field of gaseous phase sterilisation the most commonmethod to generate the vapour is by evaporating an aqueous solution ofthe sterilant on a hot surface. Typically, the solution would be 35% w/whydrogen peroxide. Such solutions tend to be unstable and give offbubbles of gas, which interfere with the conventional flow measuringsystems.

[0003] This problem was recognised and dealt with in EP O 662 844 B1, bydrawing the sterilising solution from a container into an accumulatorand measuring the weight loss in the container. Whilst this techniqueallows for a known weight of liquid to be delivered to the evaporatorand then to turn this weight of liquid into a vapour it has twoshortcomings. Firstly, it is necessary to decide at the start of theprocess how much liquid will be required, and secondly it limits theamount of liquid that may be dispensed. The apparatus described in EPO662 844 B1 requires that the liquid flow rate to the evaporator isgreater than the flow rate used to fill the accumulator. This differencein the flow rate means that the accumulator cannot be refilled from thecontainer as the flow from the accumulator will always be greater thanthe flow with the accumulator.

[0004] It is also important in gaseous sterilisation process to be ableto control the concentration of the gas being delivered to the chamberto be sterilised. The concentration will depend on the mass flow of thecarrier gas, normally air, and the rate at which liquid is evaporatedinto the air stream. The present invention not only deals with the firsttwo difficulties found in Patent EPO 662 844 B1 but also provides amethod of measuring and controlling the liquid flow to the evaporator.

[0005] An object of the present invention is to provide a method forcontinuously delivering a measured flow of liquid to a processingapparatus such as an evaporator when the liquid may be unstable andbubbles may be generated spontaneously in the liquid flow path.

[0006] The invention provides a system for continuous delivery of ameasured flow of liquid to a processing apparatus comprising a reservoirfor holding a bulk supply of the liquid, an intermediate vessel, firstmeans to pump liquid from the intermediate vessel to processingequipment at one rate, second means to pump liquid from the reservoir tothe intermediate vessel of a second much faster rate, means to monitorcontinuously the weight of the vessel and to determine the mass flowrate of liquid form the intermediate vessel and to control the firstpump means accordingly to deliver the required weight of liquid at therequired rate to the processing equipment, means to determine when theweight of liquid in the intermediate level has reached a minimumposition to initiate operation of the second pump means to refill thevessel and means to terminate operation of the second pump means whenthe liquid level in the vessel reaches a maximum position.

[0007] In one specific embodiment according to the invention, the systemconsists of a primary liquid reservoir, a measuring tube, and pumps totransfer the liquid from the reservoir to the measuring tube, and fromthe measuring tube to the liquid evaporator. A further pump may beprovided to remove any residual liquid from the measuring tube to thereservoir at the end of the sterilisation cycle. It is essential thatthe delivery rate of the pump feeding the liquid from the reservoir tothe measuring tube should be about 20 times faster than the maximumspeed of the pump delivering the liquid from the measuring tube to theevaporator. The liquid flow rate required to be delivered from themeasuring tube to the evaporator will depend on the mass flow of thecarrier gas and the required concentration of the sterilising gas.

[0008] The liquid is first pumped from the reservoir into the measuringtube. The pressure at the bottom of the measuring tube will increasewith the height of the column of liquid and is measured using a pressuretransducer.

[0009] When sufficient liquid has been delivered to the measuring tubethe pump delivering the liquid will be stopped and the system is readyto start delivering liquid to the evaporator.

[0010] As soon as it is required to start the sterilisation processliquid is pumped from the measuring tube to the evaporator, and the rateof change in the height of the column is measured using the pressuretransducer. The change of pressure may be converted into a mass flowrate from knowledge of the diameter of the measuring tube. Once thisrate of flow is known it may be used to adjust the pump speed to correctany deviations from the selected mass flow. Because this is a genuinemass flow technique it eliminates the effects of bubble formation.

[0011] When the level of the liquid in the measuring tube falls to apre-set low level the filling pump is started and the measuring tube isrefilled as before. During the refilling process the delivery pump tothe evaporator runs at the last adjusted speed and the mass flow of theliquid in the evaporator is assumed to remain constant.

[0012] At the end of the sterilisation process the control systemcalculates the total mass of liquid delivered to the evaporator from themass of the liquid delivered from the measuring tube, plus thecalculated mass flow during the time taken to re-fill the measuringtube.

[0013] The following is a description of some specific embodiments ofthe invention, reference being made to the accompanying drawings, inwhich:

[0014]FIG. 1 is a diagrammatic view of a liquid delivery systemaccording to the invention; and

[0015]FIG. 2 is a diagram showing a modified arrangement.

[0016] Referring firstly to FIG. 1 of the drawings, there is shown anapparatus for producing a measured continuous flow of liquid such aswater to a processing apparatus such as an evaporator.

[0017] The apparatus consists of a container 10 fluidly connected by apipe 11 to a column form measuring tube 12 via a pump 13. At the base ofthe measuring tube is fitted a pressure transducer 14 to measure thepressure exerted by the column of liquid in the measuring tube. A Pump15 delivers liquid to an evaporator (not shown) from the measuring tube.An overflow tube 16 is provided to connect the top of the measuring tubeto the container to return excess fluid to the container should a faultarise and the measuring tube become overfilled. A pump 17 is used topump excess liquid at the end of the sterilisation process via pipe 18,which fluidly connects the base of the measuring tube to the container.

[0018] A control system, typically based on a PLC, is used to controlthe operation of the apparatus. The pump 15 is a variable speed pumptypically a peristaltic pump, and may be used to vary the liquid flowrate from the measuring tube to the evaporator. The pump 13 is a fixedspeed pump, typically having a mass flow rate about 20 times greaterthan the maximum delivery rate of pump 15. Pump 17 is used to empty themeasuring tube at the end of the cycle and may have any convenient rate.Typically pump 13 and pump 17 are peristaltic pumps, but any other typethat is compatible with the liquid would be suitable. The liquid to beevaporated as the sterilising agent is first pumped from the containerthrough the pipe fluidly connecting the container to the measuring tubeby the pump 13, until the required mass of liquid as measured by thePressure Transducer has been transferred.

[0019] Once the measuring tube is full then the system is ready to startdelivering the liquid to the evaporator using the pump 15. The initialspeed of the pump 15 is set by the control system to give the desiredmass flow rate. The initial speed of the pump is set by reference to thedata stored in the PLC from the calibration of the pump speed and theflow rate.

[0020] Once liquid is delivered to the evaporator by pump 13, the liquidlevel in the measuring tube will start to fall, and hence the staticpressure measured by the pressure transducer will also fall. Bymonitoring the rate of fall of pressure the PLC is able to calculate theactual delivery rate achieved by pump 15. The PLC is then able to adjustthe speed of pump 15 to adjust for any deviation of the actual mass flowfrom the required mass flow.

[0021] Should the sterilisation not be completed when the liquid in themeasuring tube falls to the minimum level, then the pump 13 will bestarted by the PLC filling the measuring tube. During the period of timethat it takes for the pump 13 to fill the measuring tube the pump 15remains running at the last adjusted speed. As soon as the measuringtube is re-filled then automatic adjustment of the speed of pump 15 isresumed to maintain the required mass flow rate.

[0022] The number of times that the measuring tube may be re-filled isonly limited to the amount of liquid available in the container. Shouldvery long sterilisation processes be required it is possible toreplenish the liquid in the container either manually or with a separateautomatic system, which senses the level of liquid in container using adip tube.

[0023] At the end of the sterilisation period the pump 17 returns anyremaining liquid from the measuring tube to the container. The overflowpipe which fluidly connects the top of the measuring tube to thecontainer is provided in the event of an equipment failure and pump 13should continue to operate after the measuring tube has been filled.

[0024] Further safety features may be programmed into the PLC to ensurethat the tube is filled within a fixed period of time, to avoid theproblem of a failure of pump 13 or having an empty container.

[0025] The total mass flow may be calculated from the change in pressurein the measuring tube, and if necessary adjusted for the liquiddelivered while the measuring tube is being re-filled. This adjustmentmay be made from knowledge of the time taken to re-fill the tube and theflow rate immediately before the re-filling was started.

[0026] The measuring tube may be re-filled any number of times and eachtime this occurs an adjustment must be added to the total mass flow.

[0027] The system is particularly suitable for use in the sterilizingapparatus described and illustrated in UK Patent Application No.9922364.6.

[0028] An alternative arrangement is shown in FIG. 2, in which themeasuring tube and a pressure transducer are replaced by a reservoir 20placed on an electronic weigh scale 21. In this configuration theprocess is identical except that the signal from the weigh scalereplaces the signal from the pressure transducer, and it is essentialthat the pipes that supply and extract liquid from the reservoir do notdisturb the electronic balance. The overflow pipe may be removed andreplaced by a dip contact 22 to ensure that the reservoir 20 is notoverfilled.

1. A system for continuous delivery of a measured flow of liquid to a processing apparatus comprising a reservoir for holding a bulk supply of the liquid, an intermediate vessel, first means to pump liquid from the intermediate vessel to processing equipment at one rate, second means to pump liquid from the reservoir to the intermediate vessel of a second much faster rate, means to monitor continuously the weight of the vessel and to determine the mass flow rate of liquid form the intermediate vessel and to control the first pump means accordingly to deliver the required weight of liquid at the required rate to the processing equipment, means to determine when the weight of liquid in the intermediate level has reached a minimum position to initiate operation of the second pump means to refill the vessel and means to terminate operation of the second pump means when the liquid level in the vessel reaches a maximum position.
 2. A system as claimed in claim 1, wherein the intermediate vessel is supported on a pressure transducer for determining the weight of the vessel and any liquid therein.
 3. A system as claimed in claim 1, wherein the intermediate vessel is supported on a weighing apparatus to determine the weight of the vessel and any liquid therein.
 4. A system as claimed in any of the preceding claims, wherein the control means for initiating operation of the second pump to recharge the intermediate vessel from the reservoir deactivates the control of the first pump means in accordance with the rate of depletion of liquid from the intermediate reservoir, the first pump means running at a constant rate during the period of replenishment of the intermediate reservoir.
 5. A system as claimed in any of the preceding claims, wherein the further pump arrangement is provided to return any unused liquid in the intermediate vessel to the reservoir. 